nix-zsh-completions: Lower priority to avoid collisions with nix

Both it and nix >=2.4 install a _nix zsh completion definition. The
one in nix should take precedence, so let's lower this package's
priority.

pkgs/shells/zsh/nix-zsh-completions/default.nix

···
   meta = with lib; {
     homepage = "https://github.com/spwhitt/nix-zsh-completions";
     description = "ZSH completions for Nix, NixOS, and NixOps";
+    priority = 6; # prevent collisions with nix 2.4's built-in completions
     license = licenses.bsd3;
     platforms = platforms.all;
     maintainers = with maintainers; [ spwhitt olejorgenb hedning ma27 ];